Marine fuels are traditionally classified according to their kinematic viscosity. This is a valid criterion for oil quality as long as the oil is produced by atmospheric distillation only.
Marine fuels are classified using the “Bunker ABC” -
-Bunker A corresponds to the distillate fuel oil No. 2
-Bunker B is a No. 4 or No. 5 fuel oil
-Bunker C corresponds to the residual fuel oil No. 6
No. 6 is the most common oil, that's why "bunker fuel" is often used as a synonym for the No. 6 residual fuel oil which requires heating before the oil can be pumped.
No. 5 fuel oil is also called “navy special”. No. 5 or No. 6 also furnace fuel oil (FFO).